1. Field of the Invention
The present invention relates to a near-field light generating device for use in heat-assisted magnetic recording where a magnetic recording medium is irradiated with near-field light to lower the coercivity of the magnetic recording medium for data recording, and a method of manufacturing the same, and to a heat-assisted magnetic recording head, a head gimbal assembly, and a magnetic recording device each of which includes the near-field light generating device.
2. Description of the Related Art
Recently, magnetic recording devices such as a magnetic disk drive have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a reproducing head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a recording head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media are discrete media each made of an aggregate of magnetic fine particles, each magnetic fine particle forming a single-domain structure. A single recording bit of a magnetic recording medium is composed of a plurality of magnetic fine particles. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic fine particles must be made smaller. However, making the magnetic fine particles smaller causes the problem that the thermal stability of magnetization of the magnetic fine particles decreases with decreasing volume of the magnetic fine particles. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data recording with existing magnetic heads.
To solve the foregoing problems, there has been proposed a method so-called heat-assisted magnetic recording. This method uses a recording medium having high coercivity. When recording data, a magnetic field and heat are simultaneously applied to the area of the recording medium where to record data, so that the area rises in temperature and drops in coercivity for data recording. Hereinafter, a magnetic head for use in heat-assisted magnetic recording will be referred to as a heat-assisted magnetic recording head.
In heat-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A commonly known method for generating near-field light is to use a near-field optical probe or so-called plasmon antenna, which is a piece of metal that generates near-field light from plasmons excited by irradiation with light. U.S. Pat. No. 6,649,894 and U.S. Pat. No. 6,768,556 each disclose a method of exciting plasmons by directly irradiating the plasmon antenna with light.
However, a plasmon antenna that is directly irradiated with light to generate near-field light is known to exhibit very low efficiency of conversion of the applied light into near-field light. The energy of the light applied to the plasmon antenna is mostly reflected off the surface of the plasmon antenna, or converted into thermal energy and absorbed by the plasmon antenna. The plasmon antenna is small in volume since the size of the plasmon antenna is set to be smaller than or equal to the wavelength of the light. The plasmon antenna therefore shows a significant increase in temperature when it absorbs the thermal energy.
Such a temperature increase makes the plasmon antenna expand in volume and protrude from a medium facing surface, which is the surface of the heat-assisted magnetic recording head to face the magnetic recording medium. This causes an end of the reproducing head located in the medium facing surface to get farther from the magnetic recording medium, thereby causing the problem that a servo signal cannot be read during recording operations.
To cope with this, as described in, for example, U.S. Pat. No. 7,330,404, there has been proposed a technique in which light propagating through a waveguide is not directly applied to a plasmon antenna but is coupled with a near-field light generating element via a buffer part in a surface plasmon polariton mode to thereby excite surface plasmons on the near-field light generating element. The near-field light generating element has a near-field light generating part which is a sharp-pointed part located in the medium facing surface to generate near-field light. According to this technique, the light propagating through the waveguide is totally reflected at the interface between the waveguide and the buffer part to generate evanescent light permeating into the buffer part. The evanescent light and collective oscillations of charges on the near-field light generating element, i.e., surface plasmons, are coupled with each other to excite the surface plasmons on the near-field light generating element. In the near-field light generating element, the excited surface plasmons propagate to the near-field light generating part, and near-field light occurs from the near-field light generating part. According to this technique, since the near-field light generating element is not directly irradiated with the light propagating through the waveguide, it is possible to prevent an excessive increase in temperature of the near-field light generating element.
Now, a description will be given of the shape of the near-field light generating element and an example of arrangement of the near-field light generating element, the buffer part and the waveguide. In this example, the near-field light generating element is disposed above the top surface of the waveguide with the buffer part therebetween. The near-field light generating element has an edge part that is opposed to the waveguide with the buffer part therebetween. Typically, as viewed in a cross section parallel to the medium facing surface, the near-field light generating element is in the shape of an isosceles triangle with its vertex downward. An end of the edge part of the near-field light generating element is located in the medium facing surface. In this near-field light generating element, the end of the edge part located in the medium facing surface and its vicinity function as the near-field light generating part. In this example, surface plasmons are excited on the edge part of the near-field light generating element. The surface plasmons propagate along the edge part to reach the near-field light generating part, and the near-field light generating part generates near-field light based on the surface plasmons. According to this example, it is possible to achieve efficient propagation of the surface plasmons excited on the edge part of the near-field light generating element to the near-field light generating part.
In the foregoing near-field light generating element, the edge part is ideally formed into a linear shape by two side surfaces making contact with each other and forming a predetermined angle therebetween. In an actually fabricated near-field light generating element, however, the edge part is rounded and thereby has a cylindrical surface configuration that connects two side surfaces forming a predetermined angle therebetween. Here, the radius of curvature of the edge part having the cylindrical surface configuration will be referred to as point radius. The angle formed between the two side surfaces that are connected through the edge part will be referred to as point angle. As will be described below, the point radius and the point angle of the near-field light generating element used in a heat-assisted magnetic recording head are significant parameters that affect the characteristics of the heat-assisted magnetic recording head.
First, the point radius will be described. The point radius is a parameter that affects the spot diameter of the near-field light occurring from the near-field light generating part. In order to increase the recording density of a magnetic recording device, a smaller spot diameter is preferred for the near-field light. To reduce the spot diameter of the near-field light, a smaller point radius is preferred.
Next, the point angle will be described. To increase the use efficiency of the light propagating through the waveguide, it is important to increase the intensity of the surface plasmons to be excited on the near-field light generating element. This requires that the wave number of the evanescent light and the wave number of the surface plasmons excited on the near-field light generating element be matched with each other. The wave number of the surface plasmons excited on the near-field light generating element varies according to the shape of the near-field light generating element, or the shape of the edge part of the near-field light generating element in particular. The point angle is thus a parameter that affects the wave number of the surface plasmons excited on the near-field light generating element. Meanwhile, the wave number of the evanescent light depends on the wavelength of the light propagating through the waveguide. When typical laser light is used as the light to propagate through the waveguide, it is necessary that the wave number of the surface plasmons to be excited on the near-field light generating element be matched with the wave number of the evanescent light that is determined depending on the wavelength of the laser light. This means that there is a preferred range for the point angle.
As seen above, for the near-field light generating element having the edge part that is opposed to the waveguide with the buffer part therebetween, it is required that the point angle fall within the preferred range so as to increase the use efficiency of the light propagating through the waveguide and that the point radius be reduced so as to reduce the spot diameter of the near-field light. In actually fabricating the near-field light generating element, however, there is a problem that the point radius is difficult to reduce particularly when the point angle is somewhat large.